Fat Metabolism

Most adults have over 100,00 kcal of energy in stored fat. This is approximately 50 times more than that available from carbohydrates stored as glucose in the liver and skeletal muscle. Our body’s ability to store energy came in handy back in caveman times where food wasn’t as readily available as it is now. However, it’s 2021 and the world is still trying to lose tons of excess body fat. In this article we take a dive into the science behind how our bodies metabolize fat for energy.

Lipolysis vs Fat Oxidation

Lipolysis is a buzzword that gets thrown around a lot when the conversation surrounds fat loss. However, it is important to note that lipolysis does not always equate to fat loss. Lipolysis is the breakdown of fat molecules stored in the body. While this is an important precursor to fat loss, simply taking fat burners that increase lipolysis won’t always guarantee that they will make you lose body fat. In the right metabolic environment, stored fat molecules break down via lipolysis and then move into fat oxidation, where they then get used for energy or actual fat loss. This perfect metabolic environment would include a diet and exercise regimen geared toward a caloric deficit. However, if lipolysis happens in the absence of a caloric deficit where the body does not need to use the broken down fat molecules for energy, they become re-esterified. Simply put, they turn back into whole fat molecules. At rest, 70% of fat molecules are re-esterified while exercise suppresses re-esterification.

Lipolysis regulators

While we know that lipolysis doesn’t directly mean fat loss, it is still an important precursor to fat loss. Some common ways to increase lipolysis are to increase circulating epinephrine. Most of the time this is done during exercise and through caffeine supplementation. Meanwhile, insulin, our body’s regulatory response to elevations in blood glucose levels inhibit lipolysis.

Fat Metabolism During Rest and Exercise

Carbs and fats are always concurrently being burned for energy. However, which fuel is being burned as the predominant source is dependent on the intensity and duration of activity. Fat metabolism is predominant at lower exercise intensities, between 60-65% of your VO2max, while carbohydrates tend to be the dominant fuel source at higher intensities >70%. Likewise, the longer an exercise bout is, the more likely someone will be using fat as energy since high intensities typically can’t be sustained over longer periods of time, and due to reductions in insulin, blood glucose levels, and increased glucagon levels.

Fat Metabolism and Resistance Exercise

Unfortunately, there is a paucity in the literature of other aspects of fat metabolism during resistance training. However, a few studies have shown that fat oxidation is increased up to 40 – 120 minutes after resistance training. Understanding how our bodies burn fat and how we can manipulate diet and exercise to help us burn fat can be a major advantage as nutrition coaches and athletes.